The present invention relates to an image forming apparatus and a process cartridge, and more particularly to an image forming apparatus provided with a rotatable contact charging member and a process cartridge provided with the same. Here, the image forming apparatus is an apparatus for forming images on recording materials using an electrophotographic image formation type process, for example. Examples of the electrophotographic image forming apparatus include an electrophotographic copying machine, an electrophotographic printer (a laser beam printer, a LED printer or the like), a facsimile machine, a word processor and the like.
Here, the process cartridge is a cartridge containing as a unit at least a member to be charged (image bearing member) and a charge member, the cartridge being detachably mountable to the main assembly of the image forming apparatus.
In the field of an electrophotographic type image forming apparatus, a so-called contact-type charging means is known in which a charge member directly contacts the surface of the member to be charged to electrically charge the surface of the member to be charged, such as an electrophotographic photosensitive member.
Referring to FIG. 8, there is shown an image forming apparatus using a contact-type charging means, in the form of a laser beam printer. The printer comprises an electrophotographic photosensitive member having an organic photosensitive layer (photosensitive drum) 101 (member to be charged) driven at a predetermined process speed in the direction indicated by an arrow. The photosensitive drum is uniformly charged by the contact-type charging means in the form of a charging roller 102.
Subsequently, the photosensitive drum 101 is exposed to scanning image light by a laser beam 103 which has been ON/OFF-controlled in accordance with image information, so that an electrostatic latent image is formed on the photosensitive drum 101. The electrostatic latent image is developed or visualized into a toner image by developing means 104 disposed opposed to the photosensitive drum 101. The developing means 104 comprises a developer carrying member in the form of a developing sleeve 104a rotationally driven in the direction indicated by an arrow, and the developing means 104 contains toner 104b which is charged to a negative polarity. The developing method is a jumping developing method, for example, and reverse development is used in which the exposed portion of the image receives the toner.
The visualized toner image is transferred onto a surface of a transfer material P (recording material) fed at predetermined timing at a transfer station 106 which is constituted by the photosensitive drum 101 and a transfer roller 105 press-contacted to the photosensitive drum 101 and rotated. The transfer material P is then fed to fixing means 107 where the toner image is fixed into a permanent image, and the transfer material P is discharged to the outside of the apparatus.
A slight amount of untransferred residual toner remaining on the surface of the photosensitive drum 101 after the completion of the transfer step, is removed by cleaning means 108. The cleaning means 108 is in the form of a cleaning blade 108a, which is an elastic member made of urethane rubber contacted counterdirectionally to the surface of the photosensitive drum 101.
The photosensitive drum 101 will be described. The photosensitive drum 101 comprises a cylinder made of aluminum or the like, an organic photosensitive layer on an outer surface thereof and a drive transmitting means at one longitudinal end portion of the cylinder. The drive transmitting means may be in the form of a substantially triangular-prism-like twisted projection (coupling member) of a resin material provided at the central portion of the end of the photosensitive drum (U.S. Pat. No. 5,903,803). The projection is engageable with a substantially triangular shaped twisted hole provided at the central portion of a gear, which is driven by a motor provided in the image forming apparatus. Alternatively, the drive transmitting means may be in the form of helical gears or the like. By using such a drive transmitting means, the photosensitive drum driving can be stabilized as compared with a driving means using conventional spur gears, thereby decreasing the occurrences of non-uniformity in the pitches in resultant images, and therefore, the image quality in half-tone images is improved.
The charging roller 102 will be described in detail. The charging roller 102 comprises an electroconductive metal core, an elastic layer on the outer surface of the electroconductive metal core, a resistance layer on the outer surface thereof, and a protection layer on the outer surface thereof, for example. The opposite end portions of the metal core are supported by bearings, which are urged toward the photosensitive drum 101 by respective springs, by which the charging roller 102 is press-contacted to the photosensitive drum 101. The charging roller 102 is supplied with an oscillating voltage including an AC component and a DC component superimposed thereon from a high voltage source through a contact, a spring, a bearing, a metal core and the like, by which the surface of the photosensitive drum 101 is electrically charged to a predetermined potential.
The bearings are made of a plastic resin material. At least one of the bearings at the side supplied with the electric power, is dispersed with carbon fiber or the like in the plastic resin material during the molding, so that its resistance value is 103-104xcexa9pScm (electroconductive).
It is known that a skewed arrangement of the contact-type charging means is known in which the rotation shaft of the photosensitive drum and the rotation shaft of the charging roller are not parallel, that is, they form a crossing angle (Japanese Laid-open Patent Application 4-213474). The main purpose of the skewed arrangement is to make uniform the contact pressure of the charging roller relative to the surface of the photosensitive drum along the longitudinal direction and to make the position of the charging roller constant along the longitudinal direction.
As regards the latter purpose, further downsizing of the main assembly of the laser beam printer or the like is desired because of the recent tendency of personal use. In view of such desire, when the maximum width of the usable sheets is 216 mm, for example, the length (width) of the charging roller is as small as approximately 220 mm, for example. However, in such a case, if the position of the charging roller relative to the maximum width is deviated, improper charging occurs at either one of longitudinal end portions with the result of fog production or sheet-edge contamination due to the improper charging adjacent the corresponding lateral end of the recording material. By the skewed arrangement of the photosensitive drum and the charging roller, the charging roller is subjected to a thrust force in the longitudinal direction to a predetermined position, so the above-discussed problem can be avoided.
Here, it is desirable to minimize the length of the photosensitive drum as well as the length of the charging roller so that the main assembly of the apparatus and the process cartridge can be shortened.
It is also desired that the time period required for the charging roller to move to a predetermined position (longitudinal direction) is shortened. If the required time period is long, the charging roller is unable to move to the predetermined position prior to the start of an image formation process responsive to the instructions from a host computer or the like to the image forming apparatus, with a result of the above-described production of fog and/or the sheet-edge contamination. With the recent trend for a shortened first print time, the problem becomes significant.
Accordingly, it is a principal object of the present invention to provide a process cartridge and an image forming apparatus in which the lengths of the charge member and the member to be charged are minimized. It is another object of the present invention to provide a process cartridge and an image forming apparatus in which the lengths of the main assembly of the image forming apparatus and the process cartridge are shortened. It is a further object of the present invention to provide a process cartridge and an image forming apparatus in which the time required for the charge member to move to a predetermined position in the longitudinal direction can be shortened.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.